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So, the last three terms of my winning sequence should be _____, 13, 17, 21.

I follow this, and I always win.
Hey, what say! What is your winning strategy?

3.15 ESTIMATION

Estimation (or estimating) is the process of finding an estimate or approximation that is usable for some purpose.
Estimation involves approximating a quantity to a required accuracy.
Why do we need estimation?
Sometimes, we may not need to know the exact count of things, and an estimate is sufficient for the
purpose at hand. Estimation helps us make quick and smart guesses.
Example 1: If you have to go to another town in your car and want your car to be filled with a sufficient amount
of fuel before starting the journey, then you need an estimation of the petrol your car needs. It depends on the
distance between two towns and the mileage of your car. If the distance between two towns is 260 km and
your car gives a mileage of 10 km per litre, then the amount of petrol your car needs is 26 litres. Now, you
go to the petrol pump and get your car filled with 30 litres of petrol so that you can have a joyful ride without
worrying about the fuel in your car.
What have you done here?
You have estimated 26 to the nearest tens by rounding it off.

Let us have one more example.
Example 2: Savita is in class 6, and her class section has 33 children. There are two other sections of class
6, having 29 and 35 children respectively. So, there are exactly 97 children in class 6. Her father asks, ‘How
many children are there in your class? She replies, ‘About 100’. That’s fair enough, isn’t it?
What has she done?

She has estimated the number of children in her class to be about 100. This is called estimating to the
nearest hundred by rounding off.
Let us learn the procedure of estimation one by one:
Estimation to the Nearest Tens (By rounding off)

The numbers 1, 2, 3, and 4 are nearer to 0 than to 10. Therefore, we round off 1, 2, 3, and 4 to 0. The numbers
6, 7, and 8 are nearer to 10, therefore we round them off to 10. The number 5 is at an equal distance from 0
and 10, but it is a common practice to round it off to 10.

• 23 is closer to 20 than to 30; therefore, it is rounded off to 20.
• 28 is closer to 30 than to 20; therefore, it is rounded off to 30.
Procedure Rule: To estimate a number to the nearest tens, we observe the immediate right digit, i.e., the ones
digit. If the ones digit is 5 or more than 5, the ones digit becomes 0, and the tens digit is increased by 1. If the
ones digit is less than 5, the ones digit becomes 0, and the rest of the digits remain unchanged.

For example: We round off some numbers to the nearest tens:
<5; becomes 0 >5; becomes 0 >5; becomes 0
(i) 4954 4950 (ii) 616 620 (iii) 98 100
remains same increased by 1 increased by 1

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